Molting Stages and Gel Limbs
Posted: Thu Feb 10, 2005 3:00 pm
Originally Posted On 06/05/03 quote:Overview of Ecdysis (Molting) The process of ecdysis is a central, and nearly continuous, part in a crab’s life. A crab may spend 90% of its time getting ready to molt, molting, or recovering from a molt. There are many dangers to molting including predation, difficulty in movement as muscles have no ridged points of attachment, desiccation, and the risk of an unsuccessful attempt to exit the old exoskeleton. Eighty to 90% of arthropod deaths are related to molting. There are four phases of the molt cycle: intermolt, roecdysis, ecdysis, and postecdysis. During the intermolt, the exoskeleton is fully formed and the animal accumulates calcium and energy stores. It is the longest phase and constitutes the time between molts. Proecdysis (premolt) starts when the old exoskeleton begins to separate from the epidermis (skin), and the new exoskeleton begins to form. Calcium and other nutrient are reabsorbed from the old exoskeleton at this time and stored in the tissue of the crab. This serves the dual purpose of softening the old exoskeleton and recycling the calcium for the new exoskeleton. The muscles of the crab then shrink so that the limbs can be pulled out of the narrow joints of the old exoskeleton. At ecdysis, the old exoskeleton cracks and the crab pulls out of it backwards. The new exoskeleton continues to form and is pale and soft. Bloating with water is responsible for the increase in size after a molt. In the case of land crabs who may not have access to water directly after molting, this water comes either from the shell water, and/or from water accumulated in the blood during preecdysis. This water pressure is used to stretch the new soft exoskeleton into a larger form. After some rest, the crab eats its old exoskeleton as a source of calcium and other nutrients. Postecdysis (postmolt) occurs as the new exoskeleton hardens through the two processes of sclerotization (tanning) and calcification. Sclerotization is the chemical process where proteins form chemical bonds between each other to form a more ridged structure. Calcification is the process of putting calcium into the exoskeleton. In this phase the muscles grow back to their natural size and the excess water is lost leaving room for further growth throughout the intermolt. Ecdysis is under hormonal regulation, which responds to many external and internal ques. Gel Legs If one leg is missing, a new one will start to form from the base. It will initially be a non functional leg folded twice on itself in a protective "jelly sac" which is actually made of chitin (a sugar chain, like cellulose). The leg will begin to grow in the first growth period called "basal growth", then its growth stops until right before the crab molts, at which point there will be a dramatic increase in growth rate during the "premolt growth". When the crab molts, the gel leg sheds its jelly sac gains a normal exoskeleton and becomes functional. If more than a critical number of limbs are missing, the molt cycle will increase and the animal molts much earlier. Jelly limbs are still generated before the molt, but the period where the limbs hit a Plateau in their growth rate (between basal and premolt growth) is much shorter than if only one or a few limbs were lost. The Importance of Water Because water pressure is the driving force behind the expansion of the new exoskeleton, it is very important that hermit crabs live in a very humid environment and have access to water that is deep enough to fill their shells. Also, hermit crabs make their blood saltier during a molt to have the water gain necessary for the expansion. Thus a salt water pond is essential for the regulation of this procees as well.I hope this helps, Lisa Bibliography Ruppert E. E. and Barnes. 1994. Invertebrate Zoology 6th ed. Saunders College Publishing, Philadelphia.Stevenson J. R. 1985. Dynamics of the Integument. Pp. 2-43 in D. E. Bliss and L. H. Mantel. The Biology of Crustacea Vol. 9: Integument, Pigments, and Hormonal Processess. Academic Press, Inc. New York.